1. Technical Field
This invention relates generally to laser printers and recorders, and more particularly to laser multi-beam printers and recorders.
2. Background Art
In a typical laser printer, modulated radiation from a laser is imaged onto a receiver to produce a desired spot size. The spot is scanned in line and page directions to create a two-dimensional image.
For higher throughput, many discrete lasers are ganged together to form multiple spots on the receiver, and multiple pixels are written simultaneously. The cost of discrete lasers and the loss of efficiency in coupling to fibers has prompted the use of a monolithic array of individually modulated laser elements to produce multiple spots. However, need to individually modulate each element at high speeds greatly complicates fabrication. The current driver electronics is expensive, high power capacity of each element makes it more susceptible to thermal and electrical cross-talks, and the failure of even one element in the array makes the array useless.
Commonly assigned co-pending U.S. patent application Ser. No. 08/261,370, filed in the name of S. Sarraf on Jun. 16, 1994, describes a light modulator having a row of elements that are uniformly illuminated by light from a multi-emitter laser array. The elements of the modulator break up the light beam, and each element of the modulator is subsequently imaged on the receiver to form desired size spots. The image pixel information comes from the modulator, not the laser.
An optical system suitable for use to illuminate the modulator using direct laser modulation is described in commonly assigned co-pending U.S. patent application Ser. No. 08/283,003, filed in the names of D. Kessler et al. on Jul. 29, 1994. This optical system is not suitable for use with a spatial light modulator.
FIG. 1a illustrates the pattern of a commercially-available PLZT shutter array modulator manufactured by Minolta. This array has a structure which consists of two parallel rows of active elements. Within a row, these elements are not in immediate contact, but are spaced apart from each other by the width of the elements. This provides a 50% "fill factor." The second row of elements is displaced from the first, and is staggered, such that the elements in the second row are 90 degrees out of phase with the elements in the first row. Finally, the elements are not a simple shape (as in square or rectangular), but are parallelograms. The layout of this Minolta modulator is illustrated in FIG. 1a, with actual dimensions of the various features, including the 190 .mu.m offset between the two rows of elements.
Generally, a modulator design with a high fill factor (90-100%) is preferred so as to minimize the light loss between elements. Various modulator technologies are under development that can have rather high fill factors, such as the deformable mirrors devices developed by Texas Instruments. However, few of these modulator technologies have yet demonstrated other highly desired performance characteristics such as modulator speeds in excess of 200 kHz per element, and the ability to handle high power densities (10 kW/cm.sup.2). PLZT modulators have demonstrated these abilities successfully, but they have trouble attaining the required high fill factors as well. The Minolta array of FIG. 1a, when combined with the optical system according to the present invention, attains an effective 100% fill factor.
Modulators with similar patterns may become available. FIG. 1b shows a modulator pattern consisting of two parallel rows of elements, where the elements have a simpler rectangular shape, and the two rows of elements are in phase with each other. Such a device has been proposed by Aura Ceramics. FIG. 1c. illustrates a device with square elements, as in FIG. 1b, but with the phase difference of FIG. 1a.
A modulator such as the device of FIG. 1a. presents particularly troublesome problems to the optical design of a laser thermal printer. Two offset rows of elements with a complex shape and a 90 degree phase difference is contrary to the natural inclination of illumination systems that create a single long narrow line of light. While one could illuminate this modulator by flooding the entire area occupied by the two rows of elements, and the gap between them, that would cause an unacceptable loss of light. More optimally, the illumination could be split into two parallel and continuous lines of light, offset by the appropriate gap. However, this too would represent an unacceptable loss of the light that falls between the elements.